How can I detect hits from a prop weapon on an non-player object?

You can use a Toggle Button for this, if you don't mind the clicking sound playing.

Spawn in a Toggle Button, and use the configure tool in your Maker Pen to configure the Toggle Button.

Enable the option "Enable Projectile Collisions".

Now, spawn a Boolean chip, set it to OR mode with the configure tool, and wire up the "When Button Toggled On" and "When Button Toggled Off" pins to the inputs of the OR chip.

When the button is shot at, the OR chip will output a signal (1) on output 0, the red pin.

You can hide the toggle button inside of your targets. Ensure your targets are part of a seperate Maker Pen Object to everything else, and change their physics modes to Decoration with the configure tool.

This lets bullets travel through them so that they hit the buttons inside.